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Variable Flip Angle T1 Consistency with and without Compensating for B1+ inhomogeneity in 3T Prostate MRI

Xinran Zhong^1,2, Sepideh Shakeri¹, Dapeng Liu¹, James Sayre¹, Steven S. Raman¹, Holden H. Wu^1,2, and Kyunghyun Sung^1,2

¹Department of Radiological Sciences, University of California, Los Angeles, Los Angeles, CA, United States, ²Physics and Biology in Medicine IDP, University of California, Los Angeles, Los Angeles, CA, United States

Synopsis

Reliable pre-contrast T₁ estimation is crucial for quantitative DCE MRI. Variable flip angle is widely used for pre-contrast T₁ measurements and is sensitive to B₁⁺ inhomogeneity. Although various B₁⁺ techniques have been proposed, the application of B₁⁺ compensation is not widely accepted yet. In this study, by evaluating T₁ intra-scanner and inter-scanner consistency with and without B₁⁺ compensation, we confirmed the necessity to perform B₁⁺ compensation and a B₁⁺ estimation method named reference region variable flip angle (RR-VFA) is recommended due to its consistent T₁ estimation and wide availability.

Introduction

A reliable quantitative dynamic contrast-enhanced (DCE) MRI is beneficial for prostate cancer diagnosis as well as treatment monitoring.¹ However, the quantification process can be hindered by multiple error sources², including uncertainties in the estimation of pre-contrast T₁. Variable flip angle (VFA) T₁ estimation is commonly used for prostate DCE-MRI, but VFA T₁ is known to be sensitive to B₁⁺ inhomogeneity³. In this study, we evaluated both intra-scanner and inter-scanner variabilities of VFA T₁ estimation with and without compensating for B₁⁺ inhomogeneity in 3T prostate MRI. We used two B₁⁺ mapping techniques, one is based on saturated turbo FLASH (satTFL)⁴, recommended by the vendor, and the other is reference region VFA (RR-VFA)^5,6, a recently proposed technique that does not need an extra scan and potentially has wide availability across all MRI systems.

Methods

With IRB approval, twenty-one volunteers (27±4 years old) were prospectively recruited to assess intra- and inter-scanner variabilities. Each subject was scanned twice on two 3.0T scanners (Skyra – Scanner 1 and Prisma – Scanner 2, Siemens Healthineers) resulting in total 84 scans, as summarized in Fig. 1. The imaging protocol consisted of 2D T2-weighted (T2W) turbo spin echo (TSE) sequence, 2D satTFL sequence for B₁⁺ estimation and 3D VFA T₁ weighted sequences with a dual-echo readout for both T₁ estimation and RR-VFA B₁⁺ estimation. After processing, each scan resulted in one anatomical T2W image, two B₁⁺ maps and three T₁ images. We mainly evaluated the three T₁ maps, including VFA T₁ map (T_1,VFA), T₁ map with satTFL B₁⁺ compensation (T_1,satTFL) and T₁ map with RR-VFA B₁⁺ compensation (T_1,RR-VFA).

To account for the position mismatch between the scans for each volunteer, rigid registration was performed for each volunteer based on T2W images and applied to corresponding B₁⁺ and T₁ maps. An experienced radiologist delineated volumetric regions of interest (ROIs) on T2W images in the prostate, left pelvic muscle and right pelvic muscle. The ROIs were transferred to T₁ maps and the average T₁ values within the ROIs were recorded for later comparison.

Lin’s Concordance Correlation Coefficient (CCC) was used to evaluate both intra- and inter-scanner T₁ consistency. There were in total two pairs of intra-scanner comparisons and four pairs of inter-scanner comparisons. For each pair of comparison, CCC was calculated and then averaged based on intra- or inter-scanner comparison respectively. To visualize the T₁ estimation consistency, linear regression plots as well as Bland-Altman plots were shown for one representative pair of comparison, and the Pearson’s correlation and limits of agreements were recorded.

Results and Discussion

One example slice of T₁ within the ROIs overlaid on T2W images were shown in Fig. 2. The figure first demonstrated that the image registration between scans achieved reasonable results, making applying the same set of ROIs on four scans at the same time feasible. More importantly, T_{1, VFA} within prostate ROI exhibited large variance across four scans and T_1,VFA within the left and right pelvic muscles ROIs had inconsistent T₁ values for the same scan. T_1,satTFL and T_1,RR-VFA on the other hand achieved more consistent T₁ estimation both between scans and within the same scan.

Linear regression plots in Fig. 3 and Bland Altman plots in Fig. 4 also confirmed this observation. In Fig. 3, T_1,RR-VFA had the regression slope closer to 1 and larger Pearson’s correlation square r² compare to T_1,VFAand T_1,satTFL. Similarly, the Bland Altman plots in Fig. 4 showed that the T₁ estimation for each tissue becomes more consistent for B₁⁺ corrected T₁ maps (T_1,satTFL and T_1,RR-VFA) compared to T_1,VFA. The 95% limits of agreement of B₁⁺ corrected T₁ were narrower than those in T_1,VFA.

The averaged CCC and standard deviation were reported in Table 1. T_{1, RR-VFA} had higher CCC (0.914 for intra-scanner and 0.897 for inter-scanner comparison) compared to T_1,satTFL (0.906 for intra-scanner and 0.880 for inter-scanner comparison) and T_1,VFA (0.881 for intra-scanner and 0.795 for inter-scanner comparison).

Conclusion

T₁ estimation with B₁⁺ compensation in 3.0 T prostate MRI improved both intra-scanner and inter-scanner T₁ estimation consistency. RR-VFA B₁⁺ correction provided at least comparable performance compared to satTFL B₁⁺ correction and is recommended in clinical practice due to its wide availability and no extra scan requirement.

Acknowledgements

The study was supported by funding from the Integrated Diagnostics program of the Departments of Radiological Sciences & Pathology & Laboratory Medicine, David Geffen School of Medicine at UCLA.

References

1. Alonzi R, Padhani AR, Allen C. Dynamic contrast enhanced MRI in prostate cancer. European Journal of Radiology. 2007;63(3):335–350.

2. Huang W, Chen Y, Fedorov A, Li X, Jajamovich GH, Malyarenko DI, Aryal MP, LaViolette PS, Oborski MJ, O’Sullivan F, et al. The Impact of Arterial Input Function Determination Variations on Prostate Dynamic Contrast-Enhanced Magnetic Resonance Imaging Pharmacokinetic Modeling: A Multicenter Data Analysis Challenge. Tomography. 2016;2(1):56–66.

3. Tsai W-C, Kao K-J, Chang K-M, Hung C-F, Yang Q, Lin C-YE, Chen C-M. B1 Field Correction of T1 Estimation Should Be Considered for Breast Dynamic Contrast-enhanced MR Imaging Even at 1.5 T. Radiology. 2017;282(1):55–62.

4. Chung S, Kim D, Breton E, Axel L. Rapid B1+ mapping using a preconditioning RF pulse with turboFLASH readout. Magnetic Resonance in Medicine. 2010;64(2):439–446.

5. Sung K, Saranathan M, Daniel BL, Hargreaves BA. Simultaneous T 1 and B 1 + Mapping Using Reference Region Variable Flip Angle Imaging. Magnetic Resonance in Medicine. 2013;70(4):954–961.

6. Rangwala NA, Dregely I, Wu HH, Sung K. Optimization and evaluation of reference region variable flip angle (RR-VFA) B1+ and T 1 Mapping in the Prostate at 3T. Journal of Magnetic Resonance Imaging. 2017;45(3):751–760.

Figures

Figure 1. Experiment design summary. Each volunteer was scanned four times on two 3.0 T scanners. The volunteers were repositioned between two scans on the same scanner, and the time interval between scans on two scanners varies from same-day to 70 days. Each scan consists of three different sequences and after post-processing, we obtained one T2W image, two B₁⁺ maps and three T₁ maps to analyze.

Figure 2. T₁ value within ROIs overlaid on T2W image after registration on one representative slice. Each row represents one T₁ map, and each column represents one scan. After registration, four scans were aligned, and the same set of ROIs can be applied to four scans at the same time. With B₁⁺ compensation, T_1,satTFL and T_1,RR-VFA had more consistent T₁ value in the prostate across different scans, and the T₁ value of left and right pelvic muscles for each scan became more uniform compared to T_1,VFA.

Figure 3. Linear Regression and squared Pearson’s Correlation (r²) for both intra-scanner comparison between Scan 1 and Scan 2 on Scanner 1 (a-c) and inter-scanner comparison between Scan 1 on Scanner 1 and Scan 1 on Scanner 2 (d-f) of average T₁ within each ROI. Each encoded color indicates one ROI. T_1,RR-VFA has the highest r².

Figure 4. Bland Altman plots for both intra-scanner comparison between Scan 1 and Scan 2 on Scanner 1 (a-c) and inter-scanner comparison between Scan 1 on Scanner 1 and Scan 1 on Scanner 2 (d-f) of average T₁ within each ROI. Each encoded color indicates one ROI. T_1,satTFL and T_1,RR-VFA have a more uniform T₁ value for each tissue, and a narrower limits of agreement compared to T_1,VFA.

Table 1. Average CCC and standard deviation for intra-scanner and inter-scanner comparison for T_1,VFA, T_1,satTFL and T_1,RR-VFA. T_1,RR-VFA had the highest CCC for both intra-scanner and inter-scanner comparison.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)

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